Preheat oven to 400°F. Grease a standard muffin pan, or line with paper baking cups.
Pour mix into a mixing bowl. Make a well in the center and add oil, milk and eggs. Mix until well combined. If desired, stir in add-ins such as fruit, nuts or chocolate chips.
Spoon batter into prepared muffin pan, filling each cup nearly to the top.
Bake until muffins are golden brown, 22–25 minutes. Cool in pan for 5 minutes. Move to wire rack to cool completely.